I. Field of the Invention
The present invention is directed to a metallo organic silver based end termination composition for a ceramic capacitor and a method for terminating such a capacitor and the terminated capacitor.
II. Description of the Prior Art
Multilayer capacitors generally comprise a ceramic body such as barium titanate, a plurality of metal film electrodes such as Ag/Pd or Pd electrodes embedded in the ceramic body and end terminations of a conductive material contacting the exposed ends of alternate electrodes and adhering to the end portions of the ceramic body. Conventional end termination compositions comprise thick film pastes composed of a glass frit mixed with a metal or metal alloy such as silver or silver/palladium. The frit provides adhesion to the ceramic body and holds the metal or metal alloy particles together.
Capacitor manufacturers would for cost reasons prefer an all silver end termination. As much as 20% palladium is required to resist leaching of the silver during soldering of the capacitor chips to leads and/or circuit boards. Because of the extremely high cost of palladium, capacitor manufacturers have looked for alternate methods of terminating capacitors to reduce the overall cost of manufacture.
A newer alternate method of manufacture comprises applying a plated layer of nickel as a solder leach barrier over a pure silver end termination. According to this method, a thick film silver paste containing silver and glass frit is applied to the ends of the capacitor body and thereafter a nickel layer is plated on top of the silver layer and then the nickel is coated with a solder such as tin or tin/lead which is either applied by plating or dipping. In addition to the precious metal cost savings realized by this method, nickel barrier terminations also offer excellent solderability and leach resistance.
A nickel barrier layer becomes nearly essential for surface mounting technology (SMT) as a result of the more vigorous soldering which takes place during direct surface mounting of the capacitor chip to the circuit board. As a result of the trend toward SMT, the use of nickel-platable silver end terminations is growing much faster than other types of terminating materials.
Conventional terminating compositions generally comprise a thick film paste containing a metal such as silver or silver/palladium which rely on glass frit to bond to the ceramic capacitor body. The use of the glass frit in certain cases leads to compatibility problems with the ceramic body and may even weaken the capacitor body itself. Also, thick film materials are very rheology sensitive and must be customized to each user's applications to avoid cosmetic problems with the final fired chip. Yield during assembly in surface mount is historically related to cosmetic appearance of the termination. In addition, capacitor chips terminated with conventional end termination compositions must be tumbled to remove sharp corners which would otherwise result in thin or bare spots on the termination.